# Taking the derivative of position to get velocity

## Homework Statement

The problem is hopefully attached, I had to take a screen shot.

## Homework Equations

I understand the process of taking the derivative of position to get velocity.
*refer to derivative rules.... for example r(x)=2x^2-6x+8 therefore r(x)=4x-6

## The Attempt at a Solution

I am stuck with what to do with the "1/s"

The correct answer is 2.8m/s

Thank you so much in advance.

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## Answers and Replies

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haruspex
Science Advisor
Homework Helper
Gold Member
I think the 1/s terms are just a confusing way of showing that the constant (0.74) has units of s-1.

heartshapedbox and gracy
I may be wrong but here is what I think - To take cosine, sine etc. you need to have an angle. Clearly it is changing in the equation -as shown- with time. say at time equals to one second - (notice that second in denominator and that of time are going to vanish with each other) - the angle is equals to 0.74 (degrees/ rads/ sterdians etc.) But I need to know what is - m- and also speed is not constant - you must have been asked to find speed(*read velocity) at a particular time.

haruspex
Science Advisor
Homework Helper
Gold Member
the angle is equals to 0.74 (degrees/ rads/ sterdians etc.)
No, the .74 is a rate, radians per second, but radians are considered dimensionless, so it can be written as just s-1.

Umrao
It's the unit. Notice the m is also there, standing for "meters". But if you take the derivative you'll get an expression that still depends on "t", so I don't see how one could end up with 2.8 m/s. Velocity is a vector quantity, so, since the answer doesn't include any unit vectors, I suppose the problem asks for the speed. But we would need a given instant of time to get the numeric solution.

HallsofIvy
Science Advisor
Homework Helper

## Homework Statement

The problem is hopefully attached, I had to take a screen shot

## Homework Equations

I understand the process of taking the derivative of position to get velocity.
There's the beginning of your problem! Velocity is the derivative with respect to time
Assuming r(t) has units of "meters" then the derivative, which is the limit of the "difference quotient" has units of "meters per time" as velocity should.

*refer to derivative rules.... for example r(x)=2x^2-6x+8 therefore r(x)=4x-6

## The Attempt at a Solution

I am stuck with what to do with the "1/s"

The correct answer is 2.8m/s

Thank you so much in advance.

heartshapedbox
haruspex
Science Advisor
Homework Helper
Gold Member
There's the beginning of your problem! Velocity is the derivative with respect to time
i see no evidence that hsb was trying to do otherwise, nor for any issue over units.

What I'm not seeing is at which point in time you're supposed to calculate the velocity... The velocity changes with time, and without knowing when to 'measure' it, we can't find a numerical answer, just the formula

Thank you everyone for your help.
When I take the derivative of the position vector I get:

(-sin(0.74t)0.74)+(cos(0.74t)0.74)
Then say at time 1.5, where the answer should equal 2.8m/s I get

(-sin(0.74x1.5)0.74)+(cos(0.74x1.5)0.74)

which equals
-0.014+0.7397
=0.7257

Where am I messing up?

Chestermiller
Mentor
You are messing up in two ways. You are omitting the 3 m and the 6 m coefficients. And the i and j represent unit vectors in the x and y directions. So both the displacement and the velocity have x and y components. To get the magnitude of the velocity, you need to consider the vector sum of the two components (using the Pythagorean theorem).

heartshapedbox
Thank you Chester. I'm STILL stuck though ahah :(

new attempt:

(3)(-sin(0.74t)0.74)+(0)(cos(0.74t)+(6)(cos(0.74t)0.74)+(0)(sin0.74t)
=(3)(-sin(0.74t)0.74)+(6)(cos(0.74t)0.74)
=(3)(-sin(0.74x1.5)0.74)+(6)(cos(0.74x1.5)0.74)
=-0.04+4.44

then, velocity= sqrt(-0.04^2+4.44^2)

I am getting 4.4....not 2.8...

I am so sorry for the trouble ahah

Chestermiller
Mentor
I get -1.988 i + 1.974 j. That gives 2.8 m/s for v. Are you sure you used radians, and not degrees?

heartshapedbox
oh man...how embarrassing ahaha, thats what I was doing. :( Thank you so very much for your help! This is quite clear now. :) Have a great day!! :D Thank you!!!